1. Technical Field of the Invention
This invention relates to means for producing monolithic ceramic parts during the chemical vapor deposition (CVD) process in a shape more closing approximating the desired final shape of a discreet part, and more particularly to the use of multiple mandrel substrates mounted on rotating shafts in a CVD furnace.
2. Background
The current technology of producing monolithic ceramic parts via the chemical vapor deposition (CVD) process consists of producing large sheets of the ceramic material in the CVD furnace, from which the final ceramic part or parts are then machined. The machining process includes cutting the rough shape out of a large sheet, grinding the piece to near thickness to produce a "blank" approximating the final form but with surplus material thickness on each face, and then machining the blank to the dimensions of the final form.
When CVD materials are deposited on large flat substrates, the material thickness profile is typically non-uniform. The CVD process time must be increased to bring the low deposition rate areas up to the minimum thickness requirements for the desired parts. The higher deposition rate areas then cause the pieces cut from the sheet to require more machining time in order to be ground to the required thickness.
There is limited flexibility in the geometry of a sheet and the pieces to be cut, so there is material waste due to the layout pattern of parts. There is also surplus material in the piece pattern that is cut or ground from between and within the pieces to make the blanks, such as edges and centers of rings, which is scrapped. Multiple depositions are required for some forms and shapes.
There are also occasional problems with cracking of a large, CVD-produced plate or sheet of ceramic material during the CVD process, which can reduce the yield significantly. The combination of cracks, the scrap material from the sheet and the material that is ground off the blanks lowers the average raw material-to-product conversion ratio significantly.